These rock faces often look striped, because different layers contain different materials as local conditions changed over millions of years.
The most useful index fossils are from animals that existed for only short periods of time, and that are found over a wide area.
Radioactive elements are ones that, after a while, will spontaneously break down and turn into other, more stable elements.
When we look at a rock face, we know that the lower layers are colder.
But we can get more detailed information using these two methods.
Ammonites changed over time, so that ones from a few million years apart are never quite the same. Other examples of good index fossils include tropites, which existed only from 230 to 208 million years ago, and trilobites, which are useful in the same way as ammonites -- due to the large number of variations over time, even though they existed for a full 270 million years.
Tropites were similar to squid and octopus, except with a hard shell, and trilobites were a type of sea-based arthropod.
By looking at how much of each element there is in a rock, we can trace it back and figure out how old those layers of rock are.